In conventional natural gas and oil wells, the natural gas and/or oil product (or both) is retrieved from pockets that may be as much as 1500 feet or more below the surface of the ground. The product is often present with water under pressure in certain materials such as shale. A conventional practice for removing the gas and oil is to sink a well having a hollow outer casing and a hollow inner tube, with the outer casing being perforated in the area where the product pocket is located. The liquid oil and/or water, (sometimes containing gas) infiltrates the perforated casing and the liquid is withdrawn from the well through the tube or pump located in the tube.
The process of removing gas with a well of this nature is to let the gas and liquid seep into the outer casing through the perforated openings in the casing. When the liquid enters the casing, the gas separates from the liquid and rises off of the top of the liquid, where the gas is drawn off for use from the top of the outer casing. The infiltration of liquid into the casing proceeds until the liquid reaches its own level in the outer casing, at which time no further liquid and entrained gas will enter the outer casing. It is thus necessary to continuously pump the liquid from the outer casing in order to make room for new gas containing liquid in the casing. This pump fitted to the inner tube at the bottom of the well draws liquid from the bottom of the outer casing and delivers the waiter upwardly through the inner tube where it exits the inner tube at ground level through a liquid outlet.
The pump can be the same for both a liquid only and for a well that includes liquid and gas. The pump is a submerged pump that is actuated by a pump rod that extends upwardly to a drive mechanism located above ground level. One type of pump is a rotary pump which operates by rotating the pump rod. A so called progressive cavity pump (sometimes called a PC pump) is an industry standard rotary pump used for dewatering oil and gas wells that produce large volumes of water.
Certain problems are encountered when the rate of "pumping down" or removing liquid from the well is not proportional with the rate at which liquid enters the well. First, if the liquid is removed too slowly, the liquid will rise above the perforations in the casing and will restrict or even stop gas removal. On the other hand, if too much liquid is removed or liquid is removed at too fast a rate, other problems can occur. Removal of liquid at too fast a rate does not give gas sufficient time to separate from the liquid. This causes the pump to pump substantial volumes of gas as well as liquid up through the tube. Pumping gas requires more pump force and reduces the efficiency of the pump, causing the pump to overheat and possibly damaging or destroying the pump. In addition, pumping gas through the tube liquid outlet is undesirable. Another problem occurs if the pump in effect pumps all of the liquid out of the outer casing, allowing the pump to run dry. This is particularly disastrous with a PC pump, which has rubber components which will burn up almost immediately if a pump is permitted to run dry. Replacement of a PC pump at a depth of 1500 feet is a very expensive proposition.
A number of devices have been used for measuring liquid depth in a well casing or controlling pump shut off after the pump runs dry. These devices, however, have had drawbacks. At the present time there is no reliable way to continuously and accurately measure and control the depth of the liquid in the outer casing while maintaining optimum pump efficiently, in order to make sure that a progressive cavity pump operates at the most efficient speed and does not run dry.
An object of the present invention is to provide an improved method and apparatus for detecting the level of liquid in a well casing and controlling both the depth of liquid in the well casing and the rate of liquid removal or pump down in the well casing in order to maximize pump efficiency and gas separation from the liquid.
For purposes of this application, the liquid in the pump casing can be water or oil or a combination of both. The water can be saltwater or fresh water. The liquid can include gas or can be substantially gas-free. The liquid could include other liquids in addition to or instead of water, such as oil.